Photomasks are high precision plates containing microscopic images of electronic circuits. Photomasks are typically made from very flat pieces of quartz or glass with a layer of chrome on one side. Etched in the chrome is a portion of an electronic circuit design. This circuit design on the mask is also called “geometry.” A typical photomask used in the production of semiconductor devices is formed from a “blank” or “undeveloped” photomask. As shown in FIG. 1, a typical blank photomask 5 is comprised of three or four layers. The first layer 11 is a layer of quartz or other substantially transparent material, commonly referred to as the substrate. The next layer is typically a layer of opaque material 12, such as Cr, which often includes a third layer of antireflective material 13, such as CrO. The antireflective layer may or may not be included in any given photomask. The top layer is typically a layer of photosensitive resist material 14. Other types of photomasks are also known and used including, but not limited to, phase shift masks, embedded attenuated phase shift masks (“EAPSM”) and alternating aperture phase shift masks (“AAPSM”). These types of phase shift masks are characterized by design features including opaque regions and partially transparent regions through which the phase of light is shifted by, for example, approximately 180°. Examples of such photomasks are described in U.S. Pat. Nos. 6,682,861, U.S. Pat. No. 6,933,084, U.S. Patent Publication No. 2005-0026053 and U.S. Pat. No. 7,049,034 to Photronics, Inc., the contents of which are incorporated by reference herein.
The process of manufacturing a photomask involves many steps and can be time consuming. In this regard, to manufacturer a photomask, the desired pattern of opaque material 12 to be created on the photomask 5 is typically defined by an electronic data file loaded into an exposure system which typically scans an electron beam (E-beam) or laser beam in a raster or vector fashion across the blank photomask. One such example of a raster scan exposure system is described in U.S. Pat. No. 3,900,737 to Collier. Each unique exposure system has its own software and format for processing data to instruct the equipment in exposing the blank photomask. As the E-beam or laser beam is scanned across the blank photomask 10, the exposure system directs the E-beam or laser beam at addressable locations on the photomask as defined by the electronic data file. The areas of the photosensitive resist material that are exposed to the E-beam or laser beam become soluble while the unexposed portions remain insoluble.
In order to determine where the e-beam or laser should expose the photoresist 14 on the blank photomask 5, and where it should not, appropriate instructions to the processing equipment need to be provided, in the form of a jobdeck. In order to create the jobdeck the images of the desired pattern are broken up (or fractured) into smaller standardized shapes, e.g., rectangles and trapezoids. The fracturing process can be very time consuming. After being fractured, the image may need to be further modified by, for example, sizing the data if needed, rotating the data if needed, adding fiducial and internal reference marks, etc. Typically a dedicated computer system is used to perform the fracturing and/or create the jobdecks. The jobdeck data must then be transferred to the processing tools, to provide such tools with the necessary instructions to expose the photomask.
After the exposure system has scanned the desired image onto the photosensitive resist material 14, as shown in FIG. 2, the soluble photosensitive resist material is removed by means well known in the art, and the unexposed, insoluble photosensitive resist material 14′ remains adhered to the opaque material 13 and 12. Thus, the pattern to be formed on the photomask 5 is formed by the remaining photosensitive resist material 14′.
The pattern is then transferred from the remaining photoresist material 14′ to the photomask 5 via known etch processes to remove the antireflective material 13 and opaque materials 12 in regions which are not covered by the remaining photoresist 14′. There is a wide variety of etching processes known in the art, including dry etching as well as wet etching, and thus a wide variety of equipment is used to perform such etching. After etching is complete, the remaining photoresist material 14′ is stripped or removed and the photomask is completed, as shown in FIG. 3. In the completed photomask, the pattern as previously reflected by the remaining antireflective material 13′ and opaque materials 12′ are located in regions where the remaining photoresist 14′ remain after the soluble materials were removed in prior steps.
In order to determine if there are any unacceptable defects in a particular photomask, it is necessary to inspect the photomask. A defect is any flaw affecting the geometry. This includes undesirable chrome areas (chrome spots, chrome extensions, chrome bridging between geometry) or unwanted clear areas (pin holes, clear extensions, clear breaks). A defect can cause the circuit to be made from the photomask not to function. The entity ordering the photomask will indicate in its defect specification the size of defects that will affect its process. All defects of that size and larger must be repaired, or if they cannot be repaired, the mask must be rejected and rewritten.
Typically, automated mask inspection systems, such as those manufactured by KLA-Tencor or Applied Materials, are used to detect defects. Such automated systems direct an illumination beam at the photomask and detect the intensity of the portion of the light beam transmitted through and reflected back from the photomask. The detected light intensity is then compared with expected light intensity, and any deviation is noted as a defect. The details of one system can be found in U.S. Pat. No. 5,563,702 assigned to KLA-Tencor.
After passing inspection, a completed photomask is cleaned of contaminants. Next, a pellicle may be applied to the completed photomask to protect its critical pattern region from airborne contamination. Subsequent through pellicle defect inspection may be performed. In some instances, the photomask may be cut either before or after a pellicle is applied.
Before performing each of the manufacturing steps described above, a semiconductor manufacturer (e.g., customer) must first provide a photomask manufacturer with different types of data relating to the photomask to be manufactured. In this regard, a customer typically provides a photomask order which includes various types of information and data which are needed to manufacture and process the photomask, including, for example, data relating to the design of the photomask, materials to be used, delivery dates, billing information and other information needed to process the order and manufacture the photomask.
After the manufacturing steps described above are completed, the completed photomask is sent to a customer for use to manufacture semiconductor and other products. In particular, photomasks are commonly used in the semiconductor industry to transfer micro-scale images defining a semiconductor circuit onto a silicon or gallium arsenide substrate or wafer. The process for transferring an image from a photomask to a silicon substrate or wafer is commonly referred to as lithography or microlithography. Typically, as shown in FIG. 4, the semiconductor manufacturing process comprises the steps of deposition, photolithography, and etching. During deposition, a layer of either electrically insulating or electrically conductive material (like a metal, polysilicon or oxide) is deposited on the surface of a silicon wafer. This material is then coated with a photosensitive resist. The photomask is then used much the same way a photographic negative is used to make a photograph. Photolithography involves projecting the image on the photomask onto the wafer. If the image on the photomask is projected several times side by side onto the wafer, this is known as stepping and the photomask is called a reticle.
As shown in FIG. 5, to create an image 21 on a semiconductor wafer 20, a photomask 5 is interposed between the semiconductor wafer 20, which includes a layer of photosensitive material, and an optical system 22. Energy generated by an energy source 23, commonly referred to as a Stepper, is inhibited from passing through the areas of the photomask 5 where the opaque material is present. Energy from the Stepper 23 passes through the transparent portions of the quartz substrate 11 not covered by the opaque material 12 and the antireflective material 13. The optical system 22 projects a scaled image 24 of the pattern of the opaque material 12 and 13 onto the semiconductor wafer 20 and causes a reaction in the photosensitive material on the semiconductor wafer. The solubility of the photosensitive material is changed in areas exposed to the energy. In the case of a positive photolithographic process, the exposed photosensitive material becomes soluble and can be removed. In the case of a negative photolithographic process, the exposed photosensitive material becomes insoluble and unexposed soluble photosensitive material is removed.
After the soluble photosensitive material is removed, the image or pattern formed in the insoluble photosensitive material is transferred to the substrate by a process well known in the art which is commonly referred to as etching. Once the pattern is etched onto the substrate material, the remaining resist is removed resulting in a finished product. A new layer of material and resist is then deposited on the wafer and the image on the next photomask is projected onto it. Again the wafer is developed and etched. This process is repeated until the circuit is complete. Because, in a typical semiconductor device many layers may be deposited, many different photomasks may be necessary for the manufacture of even a single semiconductor device. Indeed, if more than one piece of equipment is used by a semiconductor manufacturer to manufacture a semiconductor device, it is possible more than one photomask may be needed, even for each layer. Furthermore, because different types of equipment may also be used to expose the photoresist in the different production lines, even the multiple identical photomask patterns may require additional variations in sizing, orientation, scaling and other attributes to account for differences in the semiconductor manufacturing equipment. Similar adjustments may also be necessary to account for differences in the photomask manufacturer's lithography equipment. These differences need to be accounted for in the photomask manufacturing process.
Conventional integrated circuit manufacturing processes using photomasks require monitoring and adjustment on a frequent basis to optimize the quality of the final product and prevent errors from occurring. In this regard, information regarding, among other things, structural features of the photomask, the integrated circuit to be manufactured using the photomask, process parameters, and performance characteristics of the photomask, must be gathered, stored and regularly updated during the manufacturing process. This often requires numerous interruptions of the overall manufacturing process to conduct steps such as simulation of the photomask performance and sampling of the integrated circuit to determine whether adjustments to the process are required. For example, a sampling procedure may reveal that one or more manufacturing tools must be adjusted to compensate for errors or defects in the integrated circuit. The time required to sample the integrated circuit and perform other monitoring steps, analyze the results, and make the necessary adjustments to the manufacturing process may result in substantial delays, thereby generating a lower yield and a reduced overall profit margin for the integrated circuit manufacturer.
Also, it is important to provide protections against unauthorized actions with a photomask, either prior to, during, or after a manufacturing process using the photomask. The photomask is generally considered to be an “unsafe” or “unsecure” medium for electronics design information. This is for three primary reasons: a) the photomask is the first physical and measurable record of the electronic design information; b) the electronics design information, once recorded on the mask, cannot be encrypted or scrambled (it is possible however to manipulate the mask content in selective ways with mask repair equipment); and c) once the photomask is recorded on a wafer through wafer exposure by an optical imaging system, the wafer pattern reveals additional measurable information about the device pattern. In the extreme case, the device can be fabricated in whole in or in part by an unauthorized source if the masks are in the parties' possession.
As an example, photomasks used to fabricate military, government or personal information sensitive devices must be assured against manipulation or copy. Also, such protections is desirable for photomasks used to fabricate commercial devices in highly competitive applications, such as, for example, dynamic random access memories (DRAMs) and flash memories. An added concern in these applications is the relative simplicity of the patterns and the amount of competitive information that can be acquired through even visual inspection of the photomasks or patterned wafers.
Protection may also be necessary for photomasks used to evaluate new equipment or test new modules of technology in a multi-company or multi-party environment. This is especially important given the pervasive use of multi-company consortia for early research and development. In particular, the initial evaluation of equipment for new process nodes requires the most advanced mask information in a nominally uncontrolled environment such as a fabrication facility for equipment owned by a third party.
Providing protection for photomasks used to generally build devices in “wafer foundries” is also important. Securing and tracking intellectual property within the IC production process remains a key concern for end users.
Actions of unauthorized users that mask owners and other interested parties may seek to prevent include exposure of the mask on a wafer by a wafer exposure system, reverse engineering of the mask content by scanning the features of the mask with an electronic imaging system such as a mask inspection system, and manipulation of the mask content for purposes of changing the electrical properties of devices being fabricated by the mask. In the case of unauthorized mask exposure, such exposure may be used to partially or completely fabricate the device intended to be manufactured using the mask, reverse engineer the device or otherwise discover the elements of the device.
Photomasks are provided with some protection against unauthorized use using several conventional methods. For example, mask information may be scanned with a mask inspection or mask measuring system and compared to an electronic version of the design database. This can be used to test against mask manipulation when compared to a verified database. However, this protection method cannot prevent unauthorized exposure of the mask or physically secure the content on the mask. Alternatively, the layers of a full device can be separated to minimize the chance of reverse engineering the device content. In this case, half the device layers might be sent to foundry A while the other half might go to foundry B. Another protection method includes escorting the mask through the process by a designated security official. In general, there is no way at present to physically validate and secure the content of a mask at the point of use.